1. Field of the Invention
The invention concerns a method for selective presentation of a movement of the lung, as well as a computer-readable medium, an image processing unit and a magnetic resonance apparatus that implements such a method.
2. Description of the Prior Art
The magnetic resonance technique (in the following “MR” stands for “magnetic resonance”) is a known technique with which images of the inside of an examination subject can be generated. Described simply, the examination subject is positioned in a strong, static, homogeneous basic magnetic field (field strengths from 0.2 Tesla to 7 Tesla or more) inside an MR apparatus so that the nuclear spins of the examination subject orient along the basic magnetic field. To excite nuclear magnetic resonances, sequences of radio-frequency excitation pulses are radiated into the examination subject, the excited nuclear magnetic resonance signals are detected, and MR images are reconstructed based thereon. The MR technique is particularly suitable for imaging of soft tissues since a particularly good contrast can be achieved.
The imaging of the lungs by means of MR has also been promoted for years, primarily in order to draw conclusions about the functioning of the lungs. However, various difficulties exist in this context. The proton densities in the region of the lung filled with air and also the proton densities of the lung parenchyma itself are very low, which leads to a poor signal-to-noise ratio (SNR). Moreover, local field changes that arise due to susceptibility changes make the MR imaging more difficult. Furthermore, the region of the lung is subject to movements due to both breathing and the heart beat of the patient to be examined, which movements lead to movement artifacts in MR images. In particular the breathing movement changes not only the position of the tissue to be acquired but also the total volume of the lung. An additional effect that hinders the qualitative evaluation of lung data is the strong, varying blood flow in the lung within a cardiac cycle.
In order to confront these difficulties, MR images of the lungs are conducted with enhancement using contrast agent such as helium or oxygen, for example. One example of oxygen-enhanced imaging is, described in Vu M. Mai et al., “Influence of Oxygen Flow Rate on Signal and T1 Changes in Oxygen-Enhanced Ventilation Imaging” in the Journal of Magnetic Resonance Imaging 16: 37-41, 2002.
Due to the short acquisition times for MR images that are able to be currently achieved, it is also possible to acquire exposures of the lungs by means of MR tomography without severe movement artifacts. Triggering of the sequence workflow with the external movement (the breathing or the heart bear), however must often ensue, or the breath must be held for the duration of the acquisition. One example of a heart beat-triggered imaging is described in Knight-Scott J. et al. “Temporal Dynamics of Blood Flow Effects in Half-Fourier Fast Spin Echo 1H Magnetic Resonance Imaging of the Human Lung” in the Journal of Magnetic Resonance Imaging 14: 411-418, 2001.
For basic field strengths of more than 0.7 T, the T2* relaxation time is very short due to susceptibility (M. Deimling in Proc. Int. Soc. Magn. Reson. Med. 8 (2000), No. 2202). Therefore spin echo-based sequences (for example single shot HASTE) are used to show the lung parenchyma since spin echo sequences suppress the influence of susceptibility changes (see again Vu M. Mai et al. in the Journal of Magnetic Resonance Imaging 16: 37-41, 2002).
In recent tomography systems, very short echo times can be achieved so that gradient echo sequences can be applied even at basic field strengths of up to 1.5 T, for example (see for example Marcus, J. T. et al. in Proc. Int. Soc. Mag. Reson. Med. (2007), No. 2777).
In evaluations of image series of the lung, for example in a temporal progression, the problem as described above occurs of the size of the lung varying due to inhaling and exhaling. This leads to a displacement of corresponding regions in the different MR images since the lung occupies different positions in different MR images. Methods are known that can fix different lung image sizes in the breathing cycle to a reference size, and with which a registration of the individual images to one another is made possible (H. G. Topf et al. in Proc. Int. Soc. Magn. Reson. Med 11 (2004) No. 671). It has therefore become possible to analyze signal changes due to density changes as a function of time. The signal-emitting volume varies due to inhaling and exhaling, such that the breathing movement leads to the signal changes. For example, ventilation defects in the lung can be made visible by the depiction of the signal changes. However, one problem that occurs is that the signal changes depend not only on the changing parenchyma density but also on the signal of the blood that propagates in and out of the bronchial vessels with the cardiac rhythm. This blood signal interferes with the analysis of the lung signal change.
A method to depict respiration patterns by a separation of the signal portions of the blood from signal portions of the lung parenchyma in a magnetic resonance image is known from DE 10 2005 010 093 A1 and includes the following steps. Magnetic resonance images of the lung are acquired in a temporal procession, the signal curve in the magnetic resonance images over time is calculated followed by a Fourier transformation of the temporal signal curve, extraction of the Fourier spectrum belonging to the lung parenchyma, and presentation of information in the magnetic resonance image that is contained in the Fourier spectrum. The acquired information from the spectrum is thereby statically superimposed on an anatomical map of the lung.
Another example of an evaluation of spectra in MR images is described by Weisskoff, R. M., Baker, J., Belliveau, J., Davis, T. L., Kwong, K. K., Cohen, M. S., & Rosen, B. R. in “Power spectrum analysis of functionally-weighted MR data: What's in the noise?”, Proceedings of the Society for Magnetic Resonance, 1, 7 (1993).